In the co-pending patent application of Donald Carleton Mead, Harvey Lee Kasdan and Jordan Lewis Dorrity entitled METHOD FOR AUTOMATIC FABRIC INSPECTION, Ser. No. 660,252 filed concurrently herewith and assigned to the same assignee as the present application, there is disclosed a basic method of fabric inspection by the analysis of diffraction patterns developed by a coherent light beam passed through the fabric.
In the method described in the foregoing application, the coherent light beam has a cross-sectional area sufficient to encompass a large number of warps and fillings making up the fabric so that a certain cross-sectional area of the fabric irradiated by the beam can be completely analyzed by appropriate detector means on the other side of the fabric. Further, if the fabric is moved in a plane normal to the direction of the beam, successive areas of the fabric can be continuously inspected, there being developed a time sequential diffraction pattern at the detector.
If the fabric material is fairly wide, a number of problems arise if it is desired to inspect the entire area of the fabric from one edge to the other. These problems primarily result from the fact that the cross-sectional area of the coherent beam is substantially smaller than the complete area of the fabric to be inspected.
If the coherent beam is expanded to cover the entire width of the web, either the energy levels fall below detectable levels or an impossibly high laser power is required. Further, the diameter of the transform lens or mirror must be as wide as the fabric thereby creating an extremely difficult lens design problem and cost. Most importantly, the beam diameter itself cannot be too large relative to the spacing of the warps and fillings in the fabric if defects are to be properly detected.
One solution would be to use a number of systems such as disclosed in the afore-mentioned patent application in side-by-side relationship to irradiate the entire width of the web. This solution is not particularly desirable since a large number of lasers, detectors and detector processing electronics would be required which is prohibitively complex and expensive.
Another possible solution would be to simply move the entire laser light beam generator from one edge of the fabric to the other while directing the beam at the fabric. However, this solution would be complicated from the mechanical standpoint and the fact that both the detector and laser beam source would have to be moved simultaneously and alignment problems would be difficult to maintain. Also, the speed of scanning in this manner would be relatively slow. On the other hand, if it is attempted to move the fabric transversely there are again encountered mechanical problems since normally a large bolt of fabric which may be hundreds of feet in length together with its transport means for moving the fabric downwardly would have to be moved. Again, any such scanning system of this type would be very slow.